Being a divorced dad who has been separated from his kids is a real issue that many men struggle to come to grips with. In the case of your husband, 9 years has passed that he can never get back. Regardless of how those circumstances came about, I am confident that barely a day would have gone past without him feeling strong emotion towards his kids – be it guilt, loss, fear of never seeing them again etc.
After 9 years his kids will have changed a lot and adapted to their circumstances. You don’t mention their ages, but if they are teens or even adults now, they will have their own lives and it can be difficult for a parent to suddenly appear and be a parent again. In fact it is unlikely to happen.
I talk about dads maintaining a connection with your kids no matter what, in my book Happy Divorced Dads. I think it is important for him to visit them and make the connection, and equally important to maintain the life he has created with you as long as you are both happy.
From my point of view, I would suggest supporting the visit with his kids, and let him have the time he needs with them, to work through the emotional issues. Let him know that you are there for him, and encourage him to maintain the contact with his kids on a regular basis when he returns.

Regards
Paul